Corrosion resistant,non-austenitic fe-ni-mo-co-si alloy

ABSTRACT

AN ALLOY CONTAINING PRINCIPALLY COBALT, SILICON, NICKEL AND MOLYBDENUM, WITH THE BALANCE IRON HAS EXCELLENT CORROSION-RESISTANT PROPERTIES WHICH RENDER IT PARTICULARLY SUITABLE IN A SEA WATER ENVIRONMENT.

United States Patent 3,782,923 CORROSION RESISTANT, NON-AUSTENITIC Fe-Ni-Mo-Co-Si ALLOY Ralph A. Mendelson, Westminster, Karl P. Standhammer, Gardena, and Roberto Valencia, Jr., Long Beach, Calit., assignors to the United States of America as represented by the Secretary of the Interior No Drawing. Filed Apr. 26, 1971, Ser. No. 137,686 Int. Cl. C21c 31/00, 39/10 US. Cl. 75--122 1 Claim ABSTRACT OF THE DISCLOSURE An alloy containing principally cobalt, silicon, nickel and molybdenum, with the balance iron has excellent corrosion-resistant properties which render it particularly suitable in a sea water environment.

BACKGROUND OF THE INVENTION The invention herein described was made under a contract with the Department of the Interior.

This invention relates to a new and improved alloy composition and more specifically to an alloy having corrosion-resistant properties, particularly in a sea water environment.

The use of alloys in a corrosive environment such as sea water requires that they possess properties of corrosion resistance while at the same time having satisfactory properties of tensile and yield strengths, good fabrication properties and that they be low cost. Two typical alloys which are currently employed for pipes in desalinization plants are a 90-10 copper-nickel and a 70- 30 copper-nickel alloy. However, the corrosion rate for these two alloys are 50 and 8 mils per year respectively and their ultimate tensile strengths are 44x10 and 55x10 p.s.i. respectively. Furthermore, since both copper and nickel tend to be in a relatively short supply, alloys formed from these two materials are fairly expensive.

It would be desirable to obtain an alloy having a corrosion rate in sea water of only about 1-3 mils per year, a yield strength about three times that of the coppernickel alloy and containing a large concentration of iron to reduce the cost of the materials employed in the alloy.

THE INVENTION It has been discovered that an alloy containing on a weight percent basis, approximately (21%) 4.8% cobalt, 38.4% nickel, 1.3% silicon, 8.9% molybdenum and the balance iron, has excellent corrosion resistance to sea water and has good yield and ultimate tensile strength. Furthermore since the main constituent in the alloy composition is iron (about 45-48%), the cost of the materials employed in the present alloy is significantly reduced.

While iron-based alloys containing large amounts of cobalt, nickel and molybdenum are well known, they are of the austenitic variety; however, the present alloy is nonaustenitic.

The corrosion resistance of the alloy was determined by first preparing rolled strips of the alloy from are cast ingots. The rolling was carried out by heating the ingot to about 1650-1675 F. under Argon and then rolling in "ice a standard cold rolling mill. The process was repeated until the rolled strip was of the desired thickness. Uncoated probes for electrical-resistance measurements were prepared from the strip and were tested in 300 F. (150 p.s.i.) natural sea water using a Magna Corrosometer. In the testing conditions used, the sea water was saturated with oxygen.

A sample of a 90-10 copper-nickel alloy was also tested in the same test cell under the same conditions. The testing was carried out for 136 hours, and from the electrical resistance measurement of the strips, the cor rosion rate of the copper-nickel alloy was calculated to be about 40-50 mils per year; by contrast, the alloy of the present invention had a corrosion rate of about 3.2 mils per year in the rolled condition and 1.3 mils per year in the welded condition.

Tensile measurements were determined from miniature tensile bars which were machined from the as-rolled strip of the alloy. The results of the miniature tensile tests were verified by employing miniature samples of 302 stainless steel as a comparison. A standard cross head (ASTM E-S, 1967) was employed at a speed of .050 inch per minute to pull the tensile specimens. The miniature test bars obtained :a yield strength of 137x10 p.s.i. and an ultimate tensile strength of 143x10 p.s.i. After heat treatment to soften the alloy, a yield strength of 30,000 p.s.i. and an ultimate tensile strength of 65,000 p.s.i. with 32% elongation was realized.

By comparison the annealed tensile and yield strengths of the 90-10 copper-nicke1 alloy were 15 10 and 44 10 p.s.i. respectively.

Accordingly, it will be seen that the alloy composition of the present invention represents a significant improvement over alloys employed in desalinization plants not only from the standpoint of corrosion resistance, but also in tensile properties and in cost. Furthermore, the alloy of the present invention can be readily fabricated and this permits it to be employed for a variety of uses such as in boilers, pipes, etc. Since its tensile properties are significantly greater than those of copper-nickel alloys, tubing thickness and hence weight can be significantly reduced. This in turn means that the structural requirements of a desalinization plant employing the alloy of the present invention can be markedly reduced. Thus, a much less expensive plant design and capital investment is required.

What is claimed is:

1. A corrosion-resistant non-austenitic alloy having the following approximate (:l%) composition:

cobalt 4.8%, silicon 1.3%, nickel 38.4%, molybdenum 8.9% and the balance iron, all parts by weight.

References Cited UNITED STATES PATENTS 1,740,880 12/1929 Smith et al -123 K 2,000,024 5/ 1935 Ide 75-123 K X 3,586,499 6/1971 Gottlieb et a1 75-123 K L. DEWAYNE RUTLEDGE, Primary Examiner US. Cl. X.R. 75123 K 

